Locating a target region in a body, for example an anatomical cavity in a body of a patient, is important, inter alia, for anesthetics, or biopsy or aspiration of material from the cavity.
For example, a regional anesthesia block of the epidural tissue-space is understood to produce effective transient anesthesia of the lower extremities of the body. It can be effectively used for a vast number of invasive procedures of the body, including but not limited to, childbirth, prosthetic hip replacement, and a variety of other surgical procedures where anesthesia below the waist is required. It can also be effectively used for treatment of chronic and acute pain including, for example, “back-pain,” ailments of the vertebrae and, compression of the accessory nerves of the spinal column. To achieve effective regional anesthesia and to block nerve transmission to the central nervous system an adequate volume of a local anesthetic solution must be deposited in close proximity to the spinal cord at a particular level of the vertebral column within the anatomic site known as the epidural space.
The epidural space is that part of the vertebral canal not occupied by the dura mater and its contents. It lies between the dura and the periosteum lining the inside of the vertebral canal. It extends from the foramen magnum to the sacral hiatus. The anterior and posterior nerve roots in the dural membrane pass across the epidural space to unite in the intervertebral bodies, and the intravertebral discs. Laterally, the epidural space is bordered by the periosteum of the vertebral pedicles, and the intervertebral foramina. Posteriorly, the bordering structures are the periosteum of the anterior surface of the laminae, the articular processes and their connecting ligaments, the periosteum of the root of the spines, and the interlaminar spaces filled by the ligamentum flavum. The space contains venous plexuses and fatty tissue which is continuous with the fat in the paravertebral space.
The epidural fluid filled space (posterior epidural space) is a limited anatomic area with an irregular shape measuring in several square millimeters with respect to the cross section of the vertebrae and spinal column. The fluid filled space is very narrow and is associated closely with the dura of the spinal column with the ligamentum flavum closely adjacent. The fluid filled space therefore has to be clearly identified when the bevel or point of the needle exits the ligamentum flavum, as the dura will be punctured if the needle continues to penetrate. The standard technique for locating the epidural fluid filled space employs the loss-of-resistance (LOR) technique. This technique utilizes a low-friction syringe made of plastic or glass connected to an epidural Touhy needle (16 to 18 gauge). In addition, other pump driven systems have been developed to identify the epidural space by utilizing pressure monitoring with visual and acoustical representation of the fluid pressure within the system or at the tip of the needle.
When using a LOR technique the needle of the syringe is advanced until the subjective “feel” of resistance by the clinician results in a distinct back-pressure on the plunger. The clinician must subjectively differentiate the back-pressure or resistance encountered to identify the location of the anatomic structure of the ligamentum flavum. The epidural fluid filled space is entered by the tip of the needle after it passes through the ligamentum flavum thus identifying a true-LOR.
During the advancement of the needle within the tissues it is common for the operator to identify a drop of pressure, or a false-LOR. The false-LOR can be attributed to the needle tip entering into a low-density tissue structure such as a vacuole (adipose tissue) or an anatomic structure with a high tissue compliance such as the interspinous tissues. Repositioning of the needle (forward and backward) occurs many times as a needle makes contact to bony vertebrae as one is attempting to find the correct trajectory to the epidural space. Any backward movement (retraction) of the needle along a path during the repositioning creates a drop in pressure in the fluid, which can result in a false-LOR further complicating the detection of a true-LOR.
Various conditions can create a false-LOR. False-LORs can lead to many problems. For example, excess fluids can be indiscriminately injected while trying to determine the location of the epidural space. The additional fluid released into these tissues can further complicate the identification of epidural space. Additionally, if the doctor has difficulty discriminating between a false-LOR and a true-LOR, the Touhy needle may be moved beyond the boundary of the epidural space and inadvertently advanced into and through the dura of the spinal cord producing what is termed a wet-tap, which can have dangerous long-term consequences to the patient.
Therefore, it is desirable to provide a system that supplements or supplants the loss of resistance information to accurately guide a needle during insertion.